U.S. patent number 5,804,152 [Application Number 08/551,918] was granted by the patent office on 1998-09-08 for method for purifying exhaust gases.
This patent grant is currently assigned to Kabushiki Kaisha Toyota Chuo Kenkyusho, Toyota Jidosha Kabushiki Kaisha. Invention is credited to Yuzo Kawai, Mareo Kimura, Naoto Miyoshi, Osamu Ogawa, Hiromasa Suzuki, Naoki Takahashi, Toshiyuki Tanaka, Koji Yokota.
United States Patent |
5,804,152 |
Miyoshi , et al. |
September 8, 1998 |
Method for purifying exhaust gases
Abstract
An automotive exhaust catalyst includes a support, which is less
likely to adsorb SO.sub.x contained in exhaust gases thereon, an
NO.sub.x storage compound loaded on the support, and a noble metal
element loaded on the support. The support is an alumina support
with a Ti--Zr composite oxide loaded thereon, or is formed of a
Ti--Zr or Ti--Zr--Y composite oxide. The composite oxide inhibits
the NO.sub.x storage compound, which is selected from alkali
metals, alkaline-earth metals and rare-earth elements, from being
poisoned by sulfur, and enhances the thermal resistance of the
support. Thus, the automotive exhaust catalyst can effectively
purify NO.sub.x contained in lean-side exhaust gases, even after it
is subjected to a thermal durability test.
Inventors: |
Miyoshi; Naoto (Nagoya,
JP), Suzuki; Hiromasa (Kasugai, JP), Ogawa;
Osamu (Toyota, JP), Kimura; Mareo (Nagoya,
JP), Kawai; Yuzo (Nisshin, JP), Yokota;
Koji (Nagoya, JP), Takahashi; Naoki (Nagoya,
JP), Tanaka; Toshiyuki (Aichi-gun, JP) |
Assignee: |
Toyota Jidosha Kabushiki Kaisha
(Toyota, JP)
Kabushiki Kaisha Toyota Chuo Kenkyusho (Aichi,
JP)
|
Family
ID: |
26338145 |
Appl.
No.: |
08/551,918 |
Filed: |
October 23, 1995 |
Foreign Application Priority Data
|
|
|
|
|
Oct 21, 1994 [JP] |
|
|
6-256976 |
Jan 13, 1995 [JP] |
|
|
7-004396 |
|
Current U.S.
Class: |
423/213.5;
423/239.1 |
Current CPC
Class: |
B01J
23/63 (20130101); B01J 23/58 (20130101); B01D
53/9481 (20130101); B01D 53/945 (20130101); Y02T
10/22 (20130101); Y02T 10/12 (20130101) |
Current International
Class: |
B01J
23/63 (20060101); B01D 53/94 (20060101); B01J
23/54 (20060101); B01J 23/58 (20060101); B01D
053/94 () |
Field of
Search: |
;423/213.5,239.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
257983 |
|
Mar 1988 |
|
EP |
|
0370523 |
|
May 1990 |
|
EP |
|
0455491 |
|
Nov 1991 |
|
EP |
|
0507590 |
|
Oct 1992 |
|
EP |
|
0573672 |
|
Dec 1993 |
|
EP |
|
3913972 |
|
Nov 1989 |
|
DE |
|
5-168860 |
|
Jul 1993 |
|
JP |
|
5-317652 |
|
Dec 1993 |
|
JP |
|
6-304476 |
|
Nov 1994 |
|
JP |
|
94/25143 |
|
Nov 1994 |
|
WO |
|
Primary Examiner: Lewis; Michael
Assistant Examiner: DiMauro; Peter
Attorney, Agent or Firm: Oliff & Berridge PLC
Claims
What is claimed is:
1. A process for purifying carbon monoxide, hydrocarbons and
nitrogen oxides at the stoichiometric point or in exhaust gases in
oxygen-rich atmosphere, thereby purifying the exhaust gases, the
process comprising the step of:
bringing exhaust gases at stoichiometric point or in oxygen-rich
atmosphere, whose oxygen concentration is more than required for
oxidizing the components to be oxidized therein, into contact with
a catalyst:
the catalyst comprising:
an alumina support;
a Ti--Zr composite oxide loaded on said alumina support;
at least one NO.sub.x storage compound selected from the group
consisting of alkali metals and alkaline-earth metals loaded on
said alumina support; and
a noble metal element loaded on said alumina support,
whereby said nitrogen oxides in said exhaust gases are adsorbed to
said NO.sub.x storage compound on said alumina support under an
oxygen-rich atmosphere in which oxygen concentration is above the
stoichiometric point that is required for oxidizing components to
be oxidized in said exhaust gases, and said adsorbed nitrogen
oxides are released and purified by a reaction with said
hydrocarbons and carbon monoxide in said exhaust gases under a
stoichiometric atmosphere or a reduction atmosphere in which oxygen
concentration is below the stoichiometric point, and
whereby said Ti--Zr composite oxide inhibits said NO.sub.x storage
compound from reacting with sulfur oxides contained in said exhaust
gases to form sulfates and sulfites.
2. The process according to claim 1, wherein said Ti--Zr composite
oxide is loaded in an amount of from 1 to 80 grams with respect to
100 grams of said alumina support.
3. The process according to claim 1, wherein said Ti--Zr composite
oxide contains Ti in a range of from 1/9 to 9/1 by molar ratio with
respect to Zr.
4. The process according to claim 1, wherein said NO.sub.x storage
compound is loaded in an amount of from 0.05 to 0.5 moles with
respect to 100 grams of said alumina support.
5. The process according to claim 1, wherein said noble metal
element is at least one element selected from the group consisting
of platinum (Pt), palladium (Pd), and rhodium (Rh).
6. The process according to claim 5, wherein said noble metal
element is at least one element selected from the group consisting
of Pt and Pd, and loaded in an amount of from 0.1 to 20.0 grams
with respect to 100 grams of said alumina support.
7. The process according to claim 5, wherein said noble metal
catalyst is Rh, and loaded in an amount of from 0.001 to 1.0 gram
with respect to 100 grams of said alumina support.
8. The process according to claim 5, wherein said Rh is loaded in
an amount of from 0.001 to 1.0 gram, and said Pt and/or said Pd is
loaded in an amount of from 0.1 to 20.0 grams with respect to 100
grams of said alumina support.
9. The process according to claim 8, wherein said Rh is loaded in a
molar ratio of 1/3 or less with respect to a loading amount of said
Pt and/or Pd.
10. The process according to claim 1, wherein said noble metal
element is loaded on said alumina support after loading said Ti--Zr
composite oxide.
11. A process for purifying carbon monoxide, hydrocarbons and
nitrogen oxides at the stoichiometric point or in exhaust gases in
oxygen-rich atmosphere, thereby purifying the exhaust gases, the
process comprising the step of:
bringing exhaust gases at the stoichiometric point or in
oxygen-rich atmosphere, whose oxygen concentration is more than
required for oxidizing the components to be oxidized therein, into
contact with a catalyst:
the catalyst comprising:
a support including a composite oxide, the composite oxide being
formed of titanium (Ti), zirconium (Zr) and yttrium (Y);
at least one NO.sub.x storage compound selected from the group
consisting of alkaline-earth metals loaded on said support; and
a noble metal element loaded on said support,
whereby said nitrogen oxides in said exhaust gases are adsorbed to
said NO.sub.x storage compound on said alumina support under an
oxygen-rich atmosphere in which oxygen concentration is above the
stoichiometric point that is required for oxidizing components to
be oxidized in said exhaust gases, and said adsorbed nitrogen
oxides are released and purified by a reaction with said
hydrocarbons and carbon monoxide in said exhaust gases under a
stoichiometric atmosphere or a reduction atmosphere in which oxygen
concentration is below the stoichiometric point, and
whereby said Ti--Zr--Y composite oxide inhibits said NO.sub.x
storage compound from reacting with sulfur oxides contained in said
exhaust gases to form sulfates and sulfites.
12. A process for purifying carbon monoxide, hydrocarbons and
nitrogen oxides at the stoichiometric point or in exhaust gases in
oxygen-rich atmosphere, thereby purifying the exhaust gases, the
process comprising the step of:
bringing exhaust gases at the stoichiometric point or in
oxygen-rich atmosphere, whose oxygen concentration is more than
required for oxidizing the components to be oxidized therein, into
contact with a catalyst:
the catalyst comprising:
a support including a composite oxide, the composite oxide being
formed of titanium (Ti) and zirconium (Zr);
at least one NO.sub.x storage compound selected from the group
consisting of alkali metals and alkaline-earth metals loaded on
said support; and
a noble metal element loaded on said support,
whereby said nitrogen oxides in said exhaust gases are adsorbed to
said NO.sub.x storage compound on said alumina support under an
oxygen-rich atmosphere in which oxygen concentration is above the
stoichiometric point that is required for oxidizing components to
be oxidized in said exhaust gases, and said adsorbed nitrogen
oxides are released and purified by a reaction with said
hydrocarbons and carbon monoxide in said exhaust gases under a
stoichiometric atmosphere or a reduction atmosphere in which oxygen
concentration is below the stoichiometric point, and
whereby said Ti--Zr composite oxide inhibits said NO.sub.x storage
compound from reacting with sulfur oxides contained in said exhaust
gases to form sulfates and sulfites.
13. The process according to claim 12, wherein said composite oxide
contains said Zr in a range of from 0.2 to 0.5 by molar ratio with
respect to said Ti and Zr.
14. The process according to claim 12, wherein said support is
coated as a carrier layer on a surface of a monolithic support
substrate, a metallic support substrate or a pellet-shaped
substrate.
15. The process according to claim 12, wherein said support forms a
monolithic support substrate or a pellet-shaped substrate.
16. The process according to claim 12, wherein said NO.sub.x
storage compound is loaded in an amount of from 0.05 to 1.0 mole
with respect to 100 grams of said support.
17. The process according to claim 12, wherein said noble metal
element is at least one element selected from the group consisting
of platinum (Pt), rhodium (Rh), palladium (Pd), gold (Au) and
silver (Ag).
18. The process according to claim 12, wherein said noble metal
element is loaded in an amount of from 0.2 to 40.0 grams with
respect to 100 grams of said support.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust
gases. Specifically, it relates to the catalyst which can
efficiently purify nitrogen oxides (NO.sub.x) included in exhaust
gases. More specifically, it relates to the catalyst which can
efficiently purify NO.sub.x included in the exhaust gases whose
oxygen concentrations are more than required for completely
oxidizing reducing components, such as carbon monoxide (CO),
hydrogen (H.sub.2) and hydrocarbons (HC), included therein. In
particular, it relates to an automotive catalyst which stores
NO.sub.x therein in fuel-lean atmosphere, and which reduces
NO.sub.x by H.sub.2 and HC in fuel-rich atmosphere.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have
been employed 3-way catalysts so far which oxidize CO and HC and
simultaneously reduce NO.sub.x. For example, the 3-way catalysts
have been known widely which comprise a thermal resistant support
formed of cordierite, a porous carrier layer formed of
gamma-alumina and disposed on the support, and a noble metal
element selected from the group consisting of platinum (Pt),
palladium (Pd) and rhodium (Rh) loaded on the carrier layer. In
particular, another 3-way catalyst has been known in which ceria
(i.e., cerium oxide) is used together with the support, the carrier
layer and the noble metal element of the former 3-way catalysts.
Ceria has an oxygen storage capacity, and improves the light-off
performance of the latter 3-way catalyst.
From the viewpoint of the global environment protection, carbon
dioxide (CO.sub.2), which is emitted from internal combustion
engines of automobiles or the like, is at issue. In order to reduce
the carbon dioxide, so-called lean-burn engines are regarded
promising. In lean-burn engines, the air-fuel mixture is
lean-burned in oxygen-rich atmosphere. The fuel consumption can be
reduced because lean-burn engines consume the fuel less.
Accordingly, the carbon dioxide, which is emitted from lean-burn
engines as one of the burned exhaust gases, is inhibited from
generating.
The conventional 3-way catalysts purify almost all CO, HC and
NO.sub.x at the stoichiometric air-fuel ratio. However, the
conventional 3-way catalysts do not have enough activity to remove
NO.sub.x under the lean condition. Thus, it has been desired to
successfully develop an automotive exhaust catalyst and a purifying
system, which can purify NO.sub.x even under the lean
condition.
Specifically, the activity of automotive exhaust catalysts depends
greatly on the air-fuel ratio (A/F) of exhaust gases. For instance,
when the A/F is larger than 14.6, i.e., when the fuel concentration
to air is lower than the stoichiometric point (or on the fuel-lean
side), the oxygen concentration is higher than the stoichiometric
point in exhaust gases. In such conditions, the conversions of CO
and HC are almost at the same level as in the stoichiometric
condition, but the conversion of NO.sub.x is rapidly decreasing
with increasing A/F. On the other hand, when the A/F is smaller
than 14.6, i.e., when the fuel concentration to air is higher than
the stoichiometric point (or on the fuel-rich side), the oxygen
concentration is lower than the stoichiometric point in exhaust
gases. In such conditions, the conversions of CO and HC are rapidly
decreasing with decreasing A/F, but the conversion of NO.sub.x is
almost at the same level as in the stoichiometric condition.
Moreover, when driving automobiles, especially when driving
automobiles in urban areas, the automobiles are accelerated and
decelerated frequently. Consequently, the air-fuel ratio varies
frequently in the range of from the values adjacent to the
stoichiometric point (air-fuel ratio: 14.6) to the fuel-rich side
(i.e., in oxygen-lean atmosphere). In order to satisfy the low fuel
consumption requirement during the driving conditions such as in
the above-described urban areas, it is necessary to operate the
automobiles on the fuel-lean side where the air-fuel mixture
containing oxygen as excessive as possible is supplied to the
engines. Hence, in view of the low fuel-consumption requirement, it
has been also desired to develop a catalyst which is capable of
adequately purifying NO.sub.x even on the fuel-lean side (i.e., in
oxygen-rich atmosphere).
Under the circumstances, the applicants of the present invention
filed the following patent applications with the Japanese Patent
Office. For example, Japanese Unexamined Patent Publication (KOKAI)
No. 5-317,652 discloses an automotive exhaust catalyst in which an
alkaline-earth metal and Pt are loaded on a porous support
including alumina, or the like. Japanese Unexamined Patent
Publication (KOKAI) No. 5-168,860 discloses an automotive exhaust
catalyst in which lanthanum (La) and Pt are loaded on a porous
support. In these catalysts, during the fuel-lean side (i.e., in
oxygen-rich atmosphere) driving, NO.sub.x is stored in the
alkaline-earth metal and lanthanum. The alkaline-earth metal and
lanthanum are hereinafter referred to as an NO.sub.x storage
compound. During the stoichiometric-point driving or the transition
area driving, which can be classified as the fuel-rich side (i.e.,
in oxygen-lean atmosphere) driving, the stored NO.sub.x reacts with
the reducing agents such as HC, CO, etc. to be purified. As a
result, these catalysts exhibit superb NO.sub.x purifying
performance during the fuel-lean side (i.e., in oxygen-rich
atmosphere) driving.
The catalyst, for instance, proposed in Japanese Unexamined Patent
Publication (KOKAI) No. 5-317,652, is believed to provide the
advantageous effect as follows; namely: the barium, one of the
alkaline-earth metals, is loaded as simple carbonate on the
support, and it reacts with NO.sub.x to produce barium nitrates,
e.g., Ba(NO.sub.3).sub.2. Thus, NO.sub.x is stored in the NO.sub.x
storage compound as the barium nitrates.
However, the exhaust gases usually contain sulfur dioxide
(SO.sub.2) gas which is produced by burning sulfur element (S)
contained in the fuel. Further, the catalyst ingredient oxidizes
SO.sub.2 to sulfur trioxide (SO.sub.3) in oxygen-rich atmosphere
(i.e., on the fuel-lean side). Still further, SO.sub.3 reacts
readily with water vapor, which is also contained in the exhaust
gases, to produce sulfite ions and sulfate ions. The sulfite ions
and sulfate ions react with the NO.sub.x storage compound to
produce sulfites and sulfates. It has been revealed that the
resulting sulfites and sulfates adversely affect the NO.sub.x
storage reaction, which is effected by the NO.sub.x storage
compound. As a result, the catalyst disclosed in the aforementioned
Japanese Patent Publications might be poisoned by sulfur to
possibly exhibit degraded purifying performance.
In particular, when the NO.sub.x storage compound is turned into
sulfites and sulfates, the NO.sub.x storage compound can hardly
store NO.sub.x therein. Consequently, the catalysts proposed in the
aforementioned Japanese Unexamined Patent Publications might suffer
from a drawback in that it exhibits the NO.sub.x purifying
performance unsatisfactorily after it is subjected to a durability
test.
In addition, the catalysts disclosed in the aforementioned Japanese
Patent Publications employ activated alumina, which has a good
adsorbing capability, as their support. The support made of
activated alumina is also likely to adsorb SO.sub.x thereon.
Accordingly, the catalysts might possibly be poisoned by sulfur
facilitatively.
To solve the aforementioned problems, the inventors of the present
invention thought of using a support formed of titania (i.e.,
titanium oxide), which is less likely to adsorb SO.sub.x thereon,
and carried out a series of experiments. According to the
experiments, the inventors found that SO.sub.x is less likely to be
adsorbed on the support made of titania, and that SO.sub.x flows to
a downstream side as it is. Thus, a catalyst including such a
support was poisoned by sulfur to a lesser extent, because only the
SO.sub.x, which contacts with a noble mental catalyst ingredient
directly, is oxidized. However, the inventors noticed that the
catalyst including the support formed of titania has the following
detrimental drawback; namely: it showed inferior initial catalytic
activities, and it kept to exhibit unsatisfactory NO.sub.x
purifying performance after it is subjected to a durability
test.
SUMMARY OF THE INVENTION
The present invention has been developed in view of the
aforementioned circumstances. It is an object of the present
invention to provide an automotive exhaust catalyst, which can
purify NO.sub.x at high conversion initially, whose NO.sub.x
storage compound is inhibited from being poisoned by sulfur, and
whose NO.sub.x purifying performance is kept from degrading even
after it is subjected to a durability test. It is another object of
the present invention to improve a support so as to be less likely
to adsorb SO.sub.x thereon, thereby providing an automotive exhaust
catalyst, which is inhibited from being poisoned by sulfur.
In accordance with the first aspect of the present invention, a
catalyst is provided which can solve the aforementioned problems.
In a first embodiment of the present invention, an automotive
exhaust catalyst comprises:
an alumina support;
a Ti--Zr composite oxide loaded on the alumina support;
at least one NO.sub.x storage compound selected from the group
consisting of alkali metals, alkaline-earth metals and rare-earth
elements, loaded on the alumina support; and
a noble metal element loaded on the alumina support.
In the first aspect, the entire Ti--Zr composite oxide loaded on
the alumina support cannot necessarily be composite oxide, but a
part thereof can be composite oxide.
In the first aspect, the Ti--Zr composite oxide is loaded on the
alumina support. The loaded Ti--Zr composite oxide can increase the
acidity of the alumina support itself. Since SO.sub.x is acidic, it
is believed that SO.sub.x is adsorbed less on the alumina support
by enlarging the acidity of the alumina support. As a result, in
the first aspect, the NO.sub.x storage compound is inhibited from
being subjected to the sulfur poisoning, which results from the
SO.sub.x adsorption.
Concerning the NO.sub.x in the exhaust gas, a majority of the
NO.sub.x is stored in the NO.sub.x storage compound, which is
disposed on the alumina support under oxygen-rich atmosphere. The
term "oxygen-rich atmosphere" means atmosphere, in which oxygen
concentrations are above a stoichiometric point that is required
for oxidizing components to be oxidized in the exhaust gas. Then,
the exhaust gas is momentarily changed from oxygen-rich to
fuel-rich, and the stored NO.sub.x is released and purified by a
reaction with HC and CO in the exhaust gas under stoichiometric
atmosphere or fuel-lean atmosphere. The term "stoichiometric
atmosphere or fuel-lean atmosphere" means atmosphere, in which
oxygen concentrations are below the stoichiometric point.
In the first aspect, the loading amount of the Ti--Zr composite
oxide preferably falls in a range of from 1 to 80 grams with
respect to 100 grams of the alumina support. When the loading
amount is less than 1 gram, the resulting automotive exhaust
catalyst is poisoned by sulfur, and exhibits degraded NO.sub.x
purifying performance after a durability test. When the loading
amount is more than 80 grams, the resulting automotive exhaust
catalyst has unsatisfactory initial NO.sub.x purifying performance,
and exhibits deteriorated oxidation activity.
The Ti--Zr composite oxide preferably contains Ti in a range of
from 1/9 to 9/1 by molar ratio with respect to Zr. When Ti and Zr
are composited outside the range, the effect (i.e., the
alumina-support-acidifying effect) resulting from the composite
oxide tends to be produced less.
In the first aspect, the NO.sub.x storage compound is loaded on the
alumina support, and is selected from the group consisting of
alkali metals, alkaline-earth metals and rare-earth elements. The
term "alkali metals" means elements of Group 1A in the periodic
table of the elements. As for the alkali metals, it is possible to
exemplify lithium (Li), sodium (Na), potassium (K), rubidium (Rb),
cesium (Cs), and francium (Fr). The term "alkaline-earth metals"
means elements of Group 2A in the periodic table of the elements.
As for the alkaline-earth metals, it is possible to exemplify
barium (Ba), beryllium (Be), magnesium (Mg), calcium (Ca), and
strontium (Sr). The term "rare-earth elements" means scandium (So),
yttrium (Y), lanthanum (La), and chemical elements with atomic
numbers 58 to 71.
The loading amount of the NO.sub.x storage compound preferably
falls in a range of from 0.05 to 0.5 moles with respect to 100
grams of the alumina support. When the loading amount is less than
0.05 moles, the resulting automotive exhaust catalyst has
deteriorated NO.sub.x purifying performance. When the loading
amount is more than 0.5 moles, the resulting automotive exhaust
catalyst exhibits degraded oxidation activity.
In the first aspect, the noble metal element is loaded on the
alumina support. The noble metal element can be at least one
element selected from the group consisting of platinum (Pt),
palladium (Pd), and rhodium (Rh). The loading amount of platinum
and/or palladium preferably falls in a range of from 0.1 to 20.0
grams, further preferably from 0.3 to 10.0 grams, with respect to
100 grams of the alumina support. When the loading amount is less
than 0.1 gram, the NO.sub.x purifying capability of the resulting
automotive exhaust catalyst is degraded initially and after a
durability test. When the loading amount is more than 20.0 grams,
not only the catalytic effect of platinum and/or palladium is
saturated, but also the excessively loaded platinum and/or
palladium cannot be utilized effectively.
Whereas, the loading amount of rhodium preferably falls in a range
of from 0.001 to 1.0 grams, further preferably from 0.05 to 0.5
grams, with respect to 100 grams of the alumina support. When the
loading amount is less than 0.001 gram, the NO.sub.x purifying
capability of the resulting automotive exhaust catalyst is degraded
initially and after a durability test. When the loading amount is
more than 1.0 gram, the thus loaded rhodium adversely affects to
deteriorate the catalytic effect of platinum and/or palladium,
which are loaded together with rhodium. It is furthermore preferred
that rhodium be used together with platinum and/or palladium.
Hence, it is preferred to relatively determine the loading amount
of rhodium with respect to the loading amount of platinum and/or
palladium. For example, the rhodium is preferably loaded in a molar
ratio of 1/3 or less, further preferably 1/5 or less, with respect
to the platinum and/or the palladium.
In the first aspect, the loading order of the Ti--Zr composite
oxide, the NO.sub.x storage compound and the noble metal element is
not specified particularly. Note that, however, it is preferred to
load the noble metal element on the alumina support after the
Ti--Zr composite oxide is loaded thereon in order to highly
disperse the noble metal element thereon.
As having been described so far, in accordance with the first
aspect, the NO.sub.x storage compound of the resulting automotive
exhaust catalyst can be inhibited from being poisoned by sulfur.
Accordingly, the resulting automotive exhaust catalyst can keep
exhibiting high NO.sub.x purifying performance even after it is
subjected to a durability test.
In a second aspect of the present invention, an automotive exhaust
catalyst comprises:
a support including a composite oxide, the composite oxide being
formed of titanium (Ti) and zirconium (Zr);
at least one NO.sub.x storage compound selected from the group
consisting of alkali metals, alkaline-earth metals and rare-earth
elements, loaded on the support; and
a noble metal element loaded on the support.
In the second aspect, the support includes the Ti--Zr composite
oxide. When the support includes composite oxide, which is formed
of titanium (Ti) and zirconium (Zr), it is less likely to adsorb
the sulfate and sulfite ions thereon than the aluminum supports
are. Even if the support adsorbs the sulfate and sulfite ions
thereon, the adsorbed sulfate and sulfite ions react with the
NO.sub.x storage compound to produce sulfates and sulfite of the
NO.sub.x storage compound, which decompose readily at low
temperature.
Concerning the NO.sub.x in the exhaust gas, a majority of the
NO.sub.x is stored in the NO.sub.x storage compound, which is
disposed on the support under oxygen-rich atmosphere. Then, the
exhaust gas is momentarily changed from oxygen-rich to fuel-rich,
and the stored NO.sub.x is released and purified by a reaction with
HC and CO in the exhaust gas under stoichiometric atmosphere or
fuel-lean atmosphere.
Thus, in the second aspect of the present automotive exhaust
catalyst, the loaded NO.sub.x storage compound and the sulfate and
sulfite ions are brought into contact with each other at reduced
probability, and the NO.sub.x storage compound is inhibited from
being poisoned by sulfur. Whereas, the NO.sub.x storage compound
and NO.sub.x are brought into contact with each other at increased
probability. Accordingly, the present automotive exhaust catalyst
is improved in terms of NO.sub.x purifying capability.
In addition, when the support is formed of the Ti--Zr composite
oxide, the support is stabilized by being composited; namely: it is
enhanced in terms of heat resistance and acidity. Hence, the
support formed of the Ti--Zr composite oxide is effective both in
the improvement of catalytic capability and in the reduction of
SO.sub.x adsorption. The thus reduced SO.sub.x adsorption
eventually results in the prevention of sulfur-poisoning.
In a third aspect of the present invention, an automotive exhaust
catalyst comprises:
a support including a composite oxide, the composite oxide being
formed of titanium (Ti), zirconium (Zr) and yttrium (Y);
at least one NO.sub.x storage compound selected from the group
consisting of alkali metals, alkaline-earth metals and rare-earth
elements, loaded on the support; and
a noble metal element loaded on the support.
Thus, in the third aspect, the support is made by further
compositing the support, employed in the second aspect, with
yttrium (Y). Hence, in the support of the third aspect, TiO.sub.2
is inhibited from transforming from the anatase type to the rutile
type. In other words, the specific surface area of the support is
controlled so as not to decrease. As a result, the automotive
exhaust catalyst of the third aspect is further improved in terms
of heat resistance.
Concerning the NO.sub.x in the exhaust gas, a majority of the
NO.sub.x is stored in the NO.sub.x storage compound, which is
disposed on the support under oxygen-rich atmosphere. Then, the
exhaust gas is momentarily changed from oxygen-rich to fuel-rich,
and the stored NO.sub.x is released and purified by a reaction with
HC and CO in the exhaust gas under stoichiometric atmosphere or
fuel-lean atmosphere.
The compositing ratio of Ti and Zr, which constitutes the support,
is not limited in particular. Note that, however, it is preferred
that the composite oxide contains the Zr in a range of from 0.2 to
0.5 by molar ratio with respect to the Ti and Zr. When the
compositing ratio falls outside the range, the support has a
reduced specific surface area, and its acidity (i.e., the number of
acidic sites) cannot increase as expected. As a result, the Ti--Zr
or Ti--Zr--Y composite oxide support cannot operate and effect the
advantages fully.
The Ti--Zr or Ti--Zr--Y composite oxide support can be coated as a
carrier layer on a surface of a monolithic support substrate, a
metallic support substrate or a pellet-shaped substrate. Moreover,
a monolithic support substrate or a pellet-shaped substrate can be
formed of the Ti--Zr or Ti--Zr--Y composite oxide support
itself.
In the second or third aspect, similarly to the first aspect, the
NO.sub.x storage compound is loaded on the Ti--Zr composite oxide
support or the Ti--Zr--Y composite oxide support, and is selected
from the group consisting of alkali metals, alkaline-earth metals
and rare-earth elements. The terms "alkali metals", "alkaline-earth
metals" and "rare-earth elements" have the same meanings as
aforementioned, and can be exemplified by the elements mentioned
earlier.
In the second or third aspect, the loading amount of the NO.sub.x
storage compound preferably falls in a range of from 0.05 to 1.0
mole with respect to 100 grams of the Ti--Zr or Ti--Zr--Y composite
oxide support. When the loading amount is less than 0.05 moles, the
overall NO.sub.x storage capacity is so low that the resulting
automotive exhaust catalyst has deteriorated NO.sub.x purifying
performance. When the loading amount is more than 1.0 mole, not
only the overall NO.sub.x storage capacity is saturated, but also
the resulting automotive exhaust catalyst purifies HC so less that
HC is emitted in an increased amount.
In the second or third aspect, the noble metal element, loaded on
the Ti--Zr or Ti--Zr--Y composite oxide support, can be at least
one element selected from the group consisting of platinum (Pt),
rhodium (Rh), palladium (Pd), gold (Au) and silver (Ag). Note that
it is especially preferred to select Pt. The loading amount of the
noble metal elements preferably falls in a range of from 0.2 to
40.0 grams, further preferably from 1.0 to 20.0 grams, with respect
to 100 grams of the Ti--Zr or Ti--Zr--Y composite oxide support.
Note that, when the loading amount of the noble metal elements is
converted to the value with respect to 1 liter of the entire volume
of the resulting automotive exhaust catalyst, it preferably falls
in a range of from 0.1 to 20.0 grams, further preferably from 0.5
to 10.0 grams. When the loading amount is less than 0.1 gram with
respect to 1 liter of the entire volume of the resulting automotive
exhaust catalyst, the resulting automotive exhaust catalyst does
not exhibit catalytic activities practically. When the loading
amount is more than 20.0 grams with respect thereto, the loaded
noble metal element does not exhibit its catalytic activities
effectively, and the resulting automotive exhaust catalyst is
little improved in terms of catalytic activities.
Similarly to the conventional automotive exhaust catalysts, the
NO.sub.x storage compound and the noble metal element can be loaded
on the Ti--Zr or Ti--Zr--Y composite oxide support by an ordinary
process, for instance, an impregnation process, a spraying process
or a slurry mixing process, by using their chlorides and
nitrates.
In accordance with the second aspect of the present invention, the
resulting automotive exhaust catalyst is extremely durable in terms
of NO.sub.x purifying performance, because the NO.sub.x storage
compound is inhibited from being poisoned by sulfur. In accordance
with the third aspect, the resulting automotive exhaust catalyst is
further improved in terms of heat resistance, and is furthermore
enhanced in terms of durability.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Having generally described the present invention, a further
understanding can be obtained by reference to the specific
preferred embodiments which are provided herein for purposes of
illustration only and are not intended to limit the scope of the
appended claims.
First Preferred Embodiment
600 grams of an alumina or active alumina powder was immersed into
1 liter of a dinitrodiammine platinum aqueous solution having a
predetermined concentration, thereby preparing a slurry. The
resulting slurry was dried to evaporate the water content, and
thereafter was calcinated at 250.degree. C. for 1 hour. Thus, a
Pt-loaded alumina or active alumina powder was prepared, in which
Pt was loaded in an amount of 2.0 grams with respect to 120 grams
of the alumina or active alumina powder.
The Pt-loaded alumina or active alumina powder was added to
2-propanol to mix, and stirred therein at 80.degree. C. for 1 hour.
While keeping on stirring the resulting mixture at 80.degree. C.,
tetraisopropyl titanate and zirconium tetra-n-butoxide was
simultaneously added to the mixture. Note that, when simultaneously
adding tetraisopropyl titanate and zirconium tetra-n-butoxide, they
were not added at one time, but were mixed and added three times
fraction by fraction. The resulting mixture was further stirred at
80.degree. C. for 2 hours, and cooled to room temperature.
Thereafter, a powder was separated from the cooled mixture by
filtration. Finally, the resulting powder was dried, and calcinated
at 500.degree. C. for 1 hour. Thus, Ti and Zr elements are loaded
on the alumina or active alumina powder as Ti--Zr composite oxide.
Note that, on the basis of the metallic conversion, Ti was loaded
in an amount of 0.48 moles with respect to 120 grams of the alumina
or active alumina powder, and Zr was loaded in an amount of 0.12
moles with respect thereto.
The resulting alumina or active alumina powder with the Pt and
Ti--Zr composite oxide loaded was charged into a barium acetate
aqueous solution having a predetermined concentration. The
resulting mixture was stirred well, and dried to evaporate the
water content. Thereafter, the residue was calcinated at
500.degree. C. for 1 hour, thereby preparing an alumina or active
alumina powder with Pt, Ti--Zr composite oxide and Ba loaded. Note
that, on the basis of the metallic conversion, Ba was loaded in an
amount of 0.30 moles with respect to 120 grams of the alumina or
active alumina powder.
970 grams of the thus prepared alumina or active alumina powder
with Pt, Ti--Zr composite oxide and Ba loaded, 680 grams of an
alumina sol including alumina in an amount of 10% by weight, and
290 grams of water were mixed, thereby preparing a slurry for
coating. Then, a plurality of honeycomb support substrates formed
of cordierite were immersed into the slurry, and thereafter each of
them was blown to blow away the slurry in excess. Thereafter, each
of the support substrates was dried, and was calcinated at 500
.degree. C. for 1 hour, thereby preparing a support having a
coating layer thereon. Note that the coating layer was formed on
the support substrate in an amount of 120 grams with respect to 1
liter of the support substrate. Thus, a plurality of automotive
exhaust catalysts were prepared. Note that, as set forth in Table 2
below, Pt was loaded on the support substrate in an amount of 2.0
grams, Ti was loaded in an amount of 0.48 moles, on the basis of
the metallic conversion, Zr was loaded in an amount of 0.12 moles,
on the basis of the metallic conversion, and Ba was loaded in an
amount of 0.30 moles, on the basis of the metallic conversion,
respectively, with respect to 1 liter of the support substrate.
Second Preferred Embodiment
Except that Ti was loaded in an amount of 0.30 moles, on the basis
of the metallic conversion, and Zr was loaded in an amount of 0.30
moles, on the basis of the metallic conversion, respectively, with
respect to I liter of the support substrate, a plurality of
automotive exhaust catalysts of the Second Preferred Embodiment
were prepared in the same manner as those of the First Preferred
Embodiment.
Third Preferred Embodiment
Except that Ti was loaded in an amount of 0.12 moles, on the basis
of the metallic conversion, and Zr was loaded in an amount of 0.48
moles, on the basis of the metallic conversion, respectively, with
respect to 1 liter of the support substrate, a plurality of
automotive exhaust catalysts of the Third Preferred Embodiment were
prepared in the same manner as those of the First Preferred
Embodiment.
Fourth through Sixth Preferred Embodiments
Except that, instead of the barium acetate aqueous solution, a
sodium nitrate aqueous solution, a potassium nitrate aqueous
solution, or a cesium nitrate aqueous solution was used, a
plurality of automotive exhaust catalysts of the Fourth through
Sixth Preferred Embodiments were prepared respectively in the same
manner as those of the First Preferred Embodiment. Note that, in
the Fourth through Sixth Preferred Embodiments, Na, K or Cs was
loaded in an amount of 0.30 moles, respectively, on the basis of
the metallic conversion, with respect to 1 liter of the support
substrate.
Seventh Preferred Embodiment
600 grams of an alumina or active alumina powder was immersed into
1 liter of a dinitrodiammine platinum aqueous solution having a
predetermined concentration, thereby preparing a slurry. The
resulting slurry was dried to evaporate the water content, and
thereafter was calcinated at 250 .degree. C. for 1 hour. Thus, a
Pt-loaded alumina or active alumina powder was prepared, in which
Pt was loaded in an amount of 2.0 grams with respect to 120 grams
of the alumina or active alumina powder.
A titania sol and a zirconia sol were added to and stirred with the
Pt-loaded alumina or active alumina powder. The resulting mixture
was dried to evaporate the water content, and was calcinated at 500
.degree. C. for 1 hour. Thereafter, in the same manner as set forth
in the First Preferred Embodiment, Ba was further loaded on the
alumina or active alumina powder with Pt and Ti--Zr composite oxide
loaded, and the resulting alumina or active alumina powder with Pt,
Ti--Zr composite oxide and Ba loaded was coated on a plurality of
honeycomb support substrates formed of cordierite to form a coating
layer thereon. Note that Ti and Zr are loaded as Ti--Zr composite
oxide on the alumina or active alumina powder wherein Ti was loaded
in an amount of 0.30 moles, on the basis of the metallic
conversion, and Zr was loaded in an amount of 0.30 moles, on the
basis of the metallic conversion, respectively, with respect to 120
grams of the alumina or active alumina powder.
Comparative Example No. 1
Except that tetraisopropyl titanate and tetra-n-butoxide zirconium
were not used, a plurality of automotive exhaust catalysts of
Comparative Example No. 1 were prepared in the same manner as those
of the First Preferred Embodiment. The resulting automotive exhaust
catalysts were naturally free from the Ti and Zr loading.
Comparative Example Nos. 2 and 3
Except that either one of tetraisopropyl titanate and
tetra-n-butoxide zirconium was used, a plurality of automotive
exhaust catalysts of Comparative Example Nos. 2 and 3 were prepared
in the same manner as those of the First Preferred Embodiment. Note
that, in the automotive exhaust catalysts of Comparative Example
No. 2, Ti was loaded in an amount of 0.60 moles, on the basis of
the metallic conversion, with respect to 1 liter of the support
substrate, and that, in the automotive exhaust catalysts of
Comparative Example No. 3, Zr was loaded in an amount of 0.60
moles, on the basis of the metallic conversion, with respect to 1
liter of the support substrate.
Comparative Example No. 4
600 grams of an alumina or active alumina powder, 185 grams of a
zirconia powder, and 120 grams of a titania powder were mixed, and
a Pt--Ti-and-Zr-loaded powder was prepared in the same manner as
set forth in the First Preferred Embodiment.
On the resulting Pt--Ti-and-Zr-loaded alumina or active alumina
powder, Ba was loaded in the same manner as set forth in the First
Preferred Embodiment. Thereafter, the resulting Pt--Ti--Zr-and-Ba
loaded alumina or active alumina powder was made into a slurry.
Finally, a plurality of honeycomb support substrates formed of
cordierite were immersed into the slurry to form a coating layer
thereon, and were made into a plurality of automotive exhaust
catalysts of Comparative Example No. 4 in the same manner as set
forth in the First Preferred Embodiment. Note that, in the
resulting automotive exhaust catalysts, Pt was loaded in an amount
of 2.0 grams, Ti was loaded in an amount of 0.30 moles, on the
basis of the metallic conversion, Zr was loaded in an amount of
0.30 moles, on the basis of the metallic conversion, and Ba was
loaded in an amount of 0.30 moles, on the basis of the metallic
conversion, respectively, with respect to 1 liter of the support
substrate.
Examination and Evaluation
Each of the automotive exhaust catalysts of the First through
Seventh Preferred Embodiments and Comparative Example Nos. 1
through 4 was disposed in a testing apparatus, and was examined for
its NO.sub.x conversion in a transition area where a rich-side
exhaust gas and a lean-side exhaust gas were flowed alternately for
2 minutes. Hereafter, the term "rich-side exhaust gas" means an
exhaust gas, which stems from the combustion of a fuel-rich
air-fuel mixture, and the term "lean-side exhaust gas" means an
exhaust gas, which stems from the combustion of a fuel-lean
air-fuel mixture. Table 1 below sets forth the compositions of the
rich-side and lean-side model exhaust gases. Note that the
rich-side and lean-side model exhaust gases were flowed at a rate
of 2 liter/minute. The results of this NO.sub.x conversion
examination are summarized as "I.C. (i.e., Initial Conversion)" in
Table 2 below. Note that the temperature of the inlet exhaust gases
was changed to three different temperatures, e.g., 250.degree. C.,
300.degree. C. and 350.degree. C. Here, the NO.sub.x conversion is
defined by the following equation:
Then, each of the automotive exhaust catalysts was subjected to a
durability test. In the durability test, a rich-side exhaust gas
including SO.sub.2 in an amount of 100 ppm was flowed for 4
minutes, and a lean-side exhaust gas including SO.sub.2 in an
amount of 100 ppm was flowed for 1 minute, thereby constituting one
degradation cycle. Each of the automotive exhaust catalyst was
exposed to 60 degradation cycles. Note that, in the durability
test, the temperature of the inlet exhaust gases was fixed at
550.degree. C. Thereafter, each of the automotive exhaust catalysts
was examined for its NO.sub.x conversion in the transition area in
the same manner as described above. The results of this NO.sub.x
conversion examination are summarized as "C.A.D.T. (i.e.,
Conversion after Durability Test)" in Table 2.
TABLE 1 ______________________________________ O.sub.2 NO C.sub.3
H.sub.6 CO H.sub.2 Composition (%) (ppm) (ppm) (%) (%) N.sub.2
______________________________________ Lean-Side Model Gas 7.86 570
1170 0.19 0.045 balance Rich-Side Model Gas 0.25 0 710 1.07 0.250
balance ______________________________________
TABLE 2
__________________________________________________________________________
Loading Amount of Metallic Component NOx Conversion (%) (with
respect to 1 liter of Support Substrate) in Transition Area Pt Ti
Zr Ba Na K Cs 250.degree. C. 300.degree. C. 350.degree. C. (gram)
(mole) (mole) (mole) (mole) (mole) (mole) I.C. C.A.D.T. I.C.
C.A.D.T. I.C. C.A.D.T.
__________________________________________________________________________
1st Pref. 2.0 0.48 0.12 0.30 -- -- -- 94 73 98 84 96 77 Embodiment
2nd Pref. 2.0 0.30 0.30 0.30 -- -- -- 95 72 98 85 96 82 Embodiment
3rd Pref. 2.0 0.12 0.48 0.30 -- -- -- 96 85 97 88 94 77 Embodiment
4th Pref. 2.0 0.30 0.30 -- 0.30 -- -- 94 69 97 80 95 72 Embodiment
5th Pref. 2.0 0.30 0.30 -- -- 0.30 -- 94 67 96 78 95 70 Embodiment
6th Pref. 2.0 0.30 0.30 -- -- -- 0.30 93 67 96 79 94 69 Embodiment
7th Pref. 2.0 0.30 0.30 0.30 -- -- -- 93 71 95 81 93 74 Embodiment
Comp. Ex. 2.0 -- -- 0.30 -- -- -- 86 60 82 65 70 60 No. 1 Comp. Ex.
2.0 0.60 -- 0.30 -- -- -- 94 61 95 70 93 64 No. 2 Comp. Ex. 2.0 --
0.60 0.30 -- -- -- 92 65 90 73 84 64 No. 3 Comp. Ex. 2.0 0.30 0.30
0.30 -- -- -- 87 66 80 72 71 65 No. 4
__________________________________________________________________________
(Note) 1. "I.C. " stands for "Initial Conversion". 2. "C.A.D.T."
stands for "Conversion after Durability Test".
It is understood from Table 2 that the NO.sub.x conversions after
the durability test, which were exhibited by the automotive exhaust
catalysts of the First through Seventh Preferred Embodiments, were
degraded less with respect to the initial NO.sub.x conversions.
Specifically, when comparing the degree of NO.sub.x conversion
degradation, the values, which were exhibited by the First through
Seventh Preferred Embodiments, were smaller than the values, which
were exhibited by Comparative Example Nos. 1 through 4. This result
implies that the automotive exhaust catalysts of the First through
Seventh Preferred Embodiments were less poisoned by sulfur during
the durability test than those of Comparative Example Nos. 1
through 4.
Comparing the First through Seventh Preferred Embodiments with
Comparative Example Nos. 1 through 4, the automotive exhaust
catalysts were inhibited less from being poisoned by sulfur when Ti
or Zr was loaded independently. Further, the automotive exhaust
catalysts were inhibited less from being poisoned by sulfur when Ti
and Zr were simultaneously loaded as independent oxides. Thus, it
is apparent that the automotive exhaust catalysts can be inhibited
from being poisoned by sulfur eventually when Ti and Zr are loaded
as composite oxide.
Eighth Preferred Embodiment
A titania (TiO.sub.2) sol and a zirconia (Zro.sub.2) sol were mixed
so that the molar ratio of Zr was 0.2 (i. e., Zr/(Ti+Zr)=0.2). The
resulting sol mixture was stirred, dried at 80.degree. C., and
calcinated at 500.degree. C. for 5 hours, thereby preparing a
powdered support, which included Ti--Zr composite oxide.
A predetermined amount of the powdered support was immersed into a
dinitrodiammine platinum aqueous solution having a predetermined
concentration. The resulting mixture was stirred for 5 hours, dried
to evaporate the water content, and calcinated at 300.degree. C. in
air for 3 hours, thereby loading platinum (Pt) on the powdered
support. The loading amount of Pt was 2.0 grams with respect to 100
grams of the powdered support. Note that 100 grams of the powdered
support is equivalent to 1 liter thereof.
Then, the powdered support with Pt loaded was immersed into a
barium acetate aqueous solution having a predetermined
concentration. The resulting mixture was stirred for 5 hours, dried
to evaporate the water content, and calcinated at 300.degree. C. in
air for 3 hours, thereby loading barium (Ba), working as the
NO.sub.x storage compound, on the Pt-loaded powdered support. The
loading amount of Ba was 0.3 moles with respect to 100 grams of the
powdered support.
Finally, the Pt-and-Ba-loaded powdered support was treated by a
hydrogen gas flow at 500.degree. C. for 3 hours, thereby preparing
a powdered automotive exhaust catalyst of the Eighth Preferred
Embodiment.
Ninth Preferred Embodiment
Except that the titania sol and the zirconia sol were mixed to
prepare a powdered support, in which the molar ratio of Zr was 0.5
(i.e., Zr/(Ti+Zr)=0.5), a powdered automotive exhaust catalyst of
the Ninth Preferred Embodiment was prepared in the same manner as
recited in the Eighth Preferred Embodiment.
Tenth Preferred Embodiment
Except that the titania sol and the zirconia sol were mixed to
prepare a powdered support, in which the molar ratio of Zr was 0.8
(i.e., Zr/(Ti+Zr)=0.8), a powdered automotive exhaust catalyst of
the Tenth Preferred Embodiment was prepared in the same manner as
recited in the Eighth Preferred Embodiment.
Eleventh Preferred Embodiment
Except that not only the titania sol and the zirconia sol but also
extra yttrium nitrate were mixed to prepare a powdered support, in
which the molar ratio of Zr was 0.2 with respect to the sum of Ti
and Zr (i.e., Zr/(Ti+Zr)=0.2), and yttrium (Y) was further included
in an amount of 10% by mole, a powdered automotive exhaust catalyst
of the Eleventh Preferred Embodiment was prepared in the same
manner as recited in the Eighth Preferred Embodiment. The powdered
support of this embodiment included Ti--Zr--Y composite oxide.
Twelfth Preferred Embodiment
Except that not only the titania sol and the zirconia sol but also
extra yttrium nitrate were mixed to prepare a powdered support, in
which the molar ratio of Zr was 0.5 with respect to the sum of Ti
and Zr (i.e., Zr/(Ti+Zr)=0.5), and yttrium (Y) was further included
in an amount of 10% by mole, a powdered automotive exhaust catalyst
of the Twelfth Preferred Embodiment was prepared in the same manner
as recited in the Eighth Preferred Embodiment.
Thirteenth Preferred Embodiment
Except that not only the titania sol and the zirconia sol but also
extra yttrium nitrate were mixed to prepare a powdered support, in
which the molar ratio of Zr was 0.8 with respect to the sum of Ti
and Zr (i.e., Zr/(Ti+Zr)=0.8), and yttrium (Y) was further included
in an amount of 10% by mole, a powdered automotive exhaust catalyst
of the Thirteenth Preferred Embodiment was prepared in the same
manner as recited in the Eighth Preferred Embodiment.
Fourteenth Preferred Embodiment
Titanium tetrachloride, zirconyl nitrate, and yttrium nitrate were
mixed and stirred so as to produce precipitate, in which the molar
ratio of Zr was 0.2 with respect to the sum of Ti and Zr (i.e.,
Zr/(Ti+Zr)=0.2), and in which yttrium (Y) was further included in
an amount of 10% by mole. Note that the precipitate was produced by
a co-precipitation process, in which urea and ammonium carbonate
were used as neutralizing agents. The resulting precipitate was
washed, dried at 80.degree. C., and calcinated at 500.degree. C.
for 5 hours, thereby preparing a powdered support, which included
Ti--Zr--Y composite oxide.
Finally, Pt and Ba were further loaded on the powdered support in
the same manner as set forth in the Eighth Preferred Embodiment,
thereby preparing a powdered automotive exhaust catalyst of the
Fourteenth Preferred Embodiment.
Fifteenth Preferred Embodiment
Except that titanium tetrachloride, zirconyl nitrate, and yttrium
nitrate were mixed and stirred so as to produce precipitate , in
which the molar ratio of Zr was 0.5 with respect to the sum of Ti
and Zr (i.e., Zr/(Ti+Zr)=0.5), and in which yttrium (Y) was further
included in an amount of 10% by mole, a powdered automotive exhaust
catalyst of the Fifteenth Preferred Embodiment was prepared in the
same manner as set forth in the Fourteenth Preferred
Embodiment.
Sixteenth Preferred Embodiment
Except that titanium tetrachloride, zirconyl nitrate, and yttrium
nitrate were mixed and stirred so as to produce precipitate, in
which the molar ratio of Zr was 0.8 with respect to the sum of Ti
and Zr (i.e., Zr/(Ti+Zr)=0.8), and in which yttrium (Y) was further
included in an amount of 10% by mole, a powdered automotive exhaust
catalyst of the Sixteenth Preferred Embodiment was prepared in the
same manner as set forth in the Fourteenth Preferred
Embodiment.
Comparative Example No. 5
A predetermined amount of an alumina powder was immersed into a
dinitrodiammine platinum aqueous solution having a predetermined
concentration. The resulting mixture was stirred for 5 hours, dried
to evaporate the water content, and calcinated at 300.degree. C. in
air for 3 hours, thereby loading platinum (Pt) on the alumina
powder. The loading amount of Pt was 2.0 grams with respect to 100
grams of the alumina powder.
Then, the alumina powder with Pt loaded was immersed into a barium
acetate aqueous solution having a predetermined concentration. The
resulting mixture was stirred for 5 hours, dried to evaporate the
water content, and calcinated at 300.degree. C. in air for 3 hours,
thereby loading barium (Ba), working as the NO.sub.x storage
compound, on the Pt-loaded alumina powder. The loading amount of Ba
was 0.3 moles with respect to 100 grams of the alumina powder.
Finally, the Pt-and-Ba-loaded alumina powder was treated by a
hydrogen gas flow at 500.degree. C. for 3 hours, thereby preparing
a powdered automotive exhaust catalyst of Comparative Example No.
5.
Comparative Example No. 6
Except that a powdered support was formed of a TiO.sub.2 powder
alone, a powdered automotive exhaust catalyst of Comparative
Example No. 6 was prepared in the same manner as recited in the
Eighth Preferred Embodiment.
Comparative Example No. 7
Except that a powdered support was formed of a ZrO.sub.2 powder
alone, a powdered automotive exhaust catalyst of Comparative
Example No. 7 was prepared in the same manner as recited in the
Eighth Preferred Embodiment.
Table 3 below summarizes the compositions, etc., of the thus
prepared powdered automotive exhaust catalysts of the Eighth
through Sixteenth Preferred Embodiments as well as Comparative
Example Nos. 5 through 7.
Examination and Evaluation
Each of the powdered automotive exhaust catalysts of the Eighth
through Sixteenth Preferred Embodiments and Comparative Example
Nos. 5 through 7 was examined for its initial NO.sub.x purifying
performance as well as its NO.sub.x purifying performance after a
durability test. Each of them was pelletized by an ordinary
process. Each of the pelletized automotive exhaust catalysts was
weighed out by 0.5 grams, disposed in a testing apparatus, and was
examined for its NO.sub.x conversion in a transition area where a
rich-side model exhaust gas and a lean-side model exhaust gas were
flowed alternately for 2 minutes. Table 4 below sets forth the
compositions of the rich-side and lean-side model exhaust gases.
Note that the rich-side and lean-side model exhaust gases were
flowed at a rate of 2 liter/minute. The results of this NO.sub.x
conversion examination are summarized as "Initial NO.sub.x
Conversion" in Table 3. Note that the temperature of the inlet
model exhaust gases was changed to three different temperatures,
e.g., 250.degree. C., 300.degree. C. and 350.degree. C. Here, the
NO.sub.x conversion is defined by the following equation:
Then, each of the pelletized automotive exhaust catalysts was
subjected to a durability test. In the durability test, a lean-side
model exhaust gas including SO.sub.2 in an amount of 400 ppm was
flowed for 4 minutes, and a rich-side model exhaust gas including
SO.sub.2 in an amount of 400 ppm was flowed for 1 minute, thereby
constituting one degradation cycle. Note that, in this durability
test, each of the pelletized automotive exhaust catalysts was
weighed out by 1 gram, and was exposed to 15 degradation cycles.
Also note that, in the durability test, the temperature of the
inlet model exhaust gases was fixed at 600.degree. C. Thereafter,
each of the pelletized automotive exhaust catalysts was examined
for its NO.sub.x conversion in the transition area in the same
manner as described above. The results of this NO.sub.x conversion
examination are summarized as "NO.sub.x Conversion after Durability
Test" in Table
TABLE 3
__________________________________________________________________________
Powdered Support Pt Ba Initial NOx Conversion Composition Loading
Loading NOx Conversion after Durability (Molar Ratio) Amount Amount
(%) Test (%) Ti Zr Y Al Zr/(Ti + Zr) (gram) (mole) 250.degree. C.
300.degree. C. 350.degree. C. 250.degree. C. 300.degree. C.
350.degree. C.
__________________________________________________________________________
8th Pref. 0.8 0.2 -- -- 0.2 2.0 0.3 90 91 88 62 51 44 Embodiment
9th Pref. 0.5 0.5 -- -- 0.5 2.0 0.3 88 90 87 61 50 45 Embodiment
10th Pref. 0.2 0.8 -- -- 0.8 2.0 0.3 87 86 82 54 48 42 Embodiment
11th Pref. 0.72 0.18 0.1 -- 0.2 2.0 0.3 89 92 90 65 56 47
Embodiment 12th Pref. 0.45 0.45 0.1 -- 0.5 2.0 0.3 89 92 91 63 52
45 Embodiment 13th Pref. 0.18 0.72 0.1 -- 0.8 2.0 0.3 85 85 82 59
50 43 Embodiment 14th Pref. 0.72 0.18 0.1 -- 0.2 2.0 0.3 94 95 88
65 56 47 Embodiment 15th Pref. 0.45 0.45 0.1 -- 0.5 2.0 0.3 95 95
89 67 57 48 Embodiment 16th Pref. 0.18 0.72 0.1 -- 0.8 2.0 0.3 88
86 83 60 54 46 Embodiment Comp. Ex. -- -- -- 1.0 -- 2.0 0.3 93 91
89 34 27 23 No. 5 Comp. Ex. 1.0 -- -- -- 0 2.0 0.3 88 90 86 40 32
24 No. 6 Comp. Ex. -- 1.0 -- -- 1.0 2.0 0.3 79 72 68 28 22 18 No. 7
__________________________________________________________________________
TABLE 4 ______________________________________ O.sub.2 NO C.sub.3
H.sub.6 CO H.sub.2 Composition (%) (ppm) (ppm) (%) (%) N.sub.2
______________________________________ Lean-Side Model Gas 7.86 570
1170 0.19 0.045 balance Rich-Side Model Gas 0.25 0 710 1.07 0.250
balance ______________________________________
It is appreciated from Table 3 that the pelletized automotive
exhaust catalysts of the Eighth through Sixteenth Preferred
Embodiments were better than those of Comparative Example Nos. 5
through 7 in terms of the NO.sub.x purifying performance after the
durability test. This advantage is believed to result from the fact
that the Ti--Zr composite oxide support is less likely to adsorb
SO.sub.x thereon than the alumina support.
Further, the pelletized automotive exhaust catalysts of the Eighth
through Tenth Preferred Embodiments were superior to those of
Comparative Example Nos. 6 and 7 in terms of the initial NO.sub.x
conversion and the NO.sub.x conversion after the durability test.
This advantage was apparently produced by making the support from
the Ti--Zr composite oxide. For instance, it is believed that the
heat resistance and acidity of the pelletized automotive exhaust
catalysts of the Eighth through Tenth Preferred Embodiments were
enhanced by the Ti--Zr composite oxide support, and the enhanced
properties resulted in the advantage.
Furthermore, it is understood that, by further compositing the
Ti--Zr composite oxide powder with yttrium, the pelletized
automotive exhaust catalysts of the Eleventh through Sixteenth
Preferred Embodiments were upgraded in terms of the NO.sub.x
conversion after the durability test. It is believed that the heat
resistance of the pelletized automotive exhaust catalysts of the
Eleventh through Sixteenth Preferred Embodiments were improved by
further compositing the Ti--Zr composite oxide support with
yttrium.
Moreover, the comparison of the test results exhibited by the
preferred embodiments reveals the following; namely: when the
Ti--Zr composite oxide support was made from the TiO.sub.2 sol and
the ZrO.sub.2 sol (e.g., Eighth through Tenth Preferred
Embodiments), the larger the molar ratio of Zr (i.e., Zr/(Ti+Zr))
was, the smaller NO.sub.x conversion the pelletized automotive
exhaust catalysts exhibited. It is apparent that an optimum result
was produced when the molar ratio of Zr fell in the range of from
0.2 to 0.5 especially. Even when the Ti--Zr--Y composite oxide
support was made by the co-precipitation process (e.g., Fourteenth
through Sixteenth Preferred Embodiments), it is similarly
appreciated that an optimum advantage was effected when the molar
ratio of Zr fell in the range of from 0.2 to 0.5 especially.
Having now fully described the present invention, it will be
apparent to one of ordinary skill in the art that many changes and
modifications can be made thereto without departing from the spirit
or scope of the present invention as set forth herein including the
appended claims.
* * * * *